First, make sure you have a pressure vacuum breaker installed where the irrigation feed leaves the house, prior to the connection to the sprinkler system. The critical-level (CL) of the vacuum breaker must be at least 12" above the highest point of irrigation piping. If this cannot be done (due to elevation), an RPZ is required. This protects your drinking water.
Secondly, a DCIAV (double check, intermediate atmospheric vent) style vacuum breaker would prevent the water on the house side of the system from migrating back to the irrigation system. An RPZ could also be used. Both the RPZ and the DCIAV provide an atmospheric break in the piping, meaning there is virtually no possibility for back siphonage.
Your problem sounds to be as one of inadequate supply for everything you are trying to do. I would surely hope you don't have a pump for the irrigation system, as that would explain a lot. Perhaps reducing the size of your zones (adding more, making each zone 'smaller') is your best bet. You most likely are exceding the flow rate of your well vs demand on your larger zones. Please don't ignore this part, as it is most likely the crux of your issue.
With all the examples above, and what you've already done, your system is 'closed' and requires a thermal expansion tank for your water heater. Make sure you have one.